This invention relates to high pressure pumps and in particular to mechanical linkages connecting the pistons and the piston actuators used in dewatering pumps for deep ocean vehicles and equipment. Such pumps must be able to operate in silt laden salt water and must be able to generate the high pressures which are required to pump this water out of deeply submerged vehicles. The prior art method of fabricating pistons for such pumps is to machine a core from stainless steel and to plasma spray a ceramic coating over this core. The steel core provides the required structural strength for the piston and the ceramic coating provides the required wear resistance. The high difference between the coefficients of thermal expansion of the steel core and the ceramic coating tends to cause the ceramic to chip. Quite often such pistons are used with cylinders which have ceramic linings. The high thermal expansion rate of the stainless steel core of the piston relative to the thermal expansion rate of the ceramic cylinder lining can cause the piston to seize in the cylinder since the clearence between the piston and its lining is extremely small. Any thermal expansion of the stainless steel core of the piston which is not accompanied by an equivalent expansion of the cylinder and its lining will decrease the small clearence and promote siezing. An inner layer of nickel is usually deposited on the stainless steel core of the piston to promote a good mechanical bond between the steel and the ceramic. The ceramic material is then embedded in the nickel. In time, the corrosive sea water attacks the nickel and weakens the bond holding the ceramic in place. Eventually this weakening will cause large pieces of ceramic to break off the piston.
These problems with the prior art pistons could be avoided if a solid ceramic piston were used instead of a ceramic coated steel piston. Such solid ceramic pistons would have a greatly increased life time over the prior art pistons because the bond between the ceramic and the steel tends to fail long before the ceramic itself is worn out of tolerance.
A solid ceramic piston will not break easily when it is subjected only to tension or compression forces along the main axis of the piston. However, since ceramic materials tend to be very brittle, the solid ceramic pistons can be easily broken by bending forces applied transversely to their main axises. Such bending forces on the piston are difficult to avoid because it is difficult to align the pistons actuators in such a way that the actuator moves along exactly the same axis as does the piston. Two types of piston actuator misalignment are possible. The axis along which the piston moves and the axis along which the piston actuator moves may be parallel to each other but nevertheless misaligned because they are offset from each other. The piston and the piston actuator may also be misaligned if the axis along which the piston actuator moves is not parallel to the axis along which the piston moves. The inventors have found that this type of nonparallel misalignment is easier to avoid when fabricating a pump than is the other type of misalignment where the two axises are parallel but offset from each other. A linkage is needed to connect a piston actuator with a solid ceramic piston in such a way that no significant bending forces will be applied to the piston even though the actuator and the piston move along axises which are parallel but slightly offset from each other.